1. Field of the Invention
The present invention relates to an ingot manufacturing apparatus and related method of manufacturing the ingot. More particularly, the invention concerns a quartz glass ingot manufacturing apparatus and method.
2. Discussion of the Related Art
It has been proposed that synthetic quartz glass and fluoride single crystals, such as fluorite, be used in place of conventional optical glasses in the lenses of illumination optical systems or projection optical systems of reduction exposure apparatus that expose and transfer fine patterns of integrated circuits onto wafers consisting of silicon and the like. Such quartz glass materials utilized in optical systems of reduction exposure apparatus must have a high transmissivity with respect to ultraviolet light as well as a high degree of refractive index uniformity. For example, when quartz glass is used as a projection lens material in photolithography, the glass must be free of striae in all three directions. Moreover, the refractive index uniformity .DELTA.n must be 4.times.10.sup.-6 or less in the direction of the optical axis; the refractive index distribution in the direction of the optical axis of the lens must show center symmetry; the RMS value of the rotational non-symmetrical element must be 0.0050 .lambda. or less; and the RMS value of the rotational symmetrical 2nd/4th-order residual must be 0.0050 .lambda. or less. Furthermore, in order to realize a high transmissivity with respect to ultraviolet light, it is necessary to suppress the concentration of impurities in the quartz glass.
For this purpose, an apparatus has been proposed that is equipped with a furnace having an opening part in the bottom, a target surface that faces this opening part, and a burner used for quartz glass synthesis in which quartz glass is manufactured by a flame hydrolysis method by which an Si compound gas constituting the raw material of the quartz glass and a combustion gas used for heating are caused to flow out from the burner, and in which soot is deposited with the flame of the burner. Since the admixture of impurities can easily be suppressed using this flame hydrolysis method, quartz glass with a high purity can be obtained.
However, while the concentration of impurities can be suppressed when quartz glass is manufactured by this flame hydrolysis method, the glass produced is often unsatisfactory because of striae generated in the finished product as a result of a non-uniform temperature distribution and with regard to the uniformity of the refractive index in the direction of the optical axis. Here, the uniformity of the refractive index depends on the temperature distribution in the direction of the diameter of the ingot when the ingot is formed on the target. Accordingly, a quartz glass manufacturing apparatus has been proposed in which the target is caused to rotate in order to adjust the temperature distribution of the head portion of the ingot so that the uniformity of the refractive index is optimized. Also, in this apparatus the burner and ingot are caused to undergo relative planar motion in accordance with the temperature distribution of the head portion of the ingot (Japanese Patent Application Kokai No. 6-234531). In this apparatus, a temperature distribution that optimizes the uniformity of the refractive index is formed. As a result, quartz glass with an improved uniformity can be obtained.
This quartz glass manufacturing apparatus has an exhaust port that is used to exhaust the chlorine gas, and the like, that is generated during the synthesis of the ingot. Secondary air used for exhaust purposes is supplied from the opening and shutting system formed in the bottom of the furnace.
In the above-mentioned quartz glass manufacturing apparatus, the ingot synthesized on the target surface becomes larger in size as time passes during the manufacturing process. As a result, the opening area of the opening and shutting system formed in the bottom of the furnace becomes smaller as the ingot gets larger. When the area of the opening and shutting system is reduced in this way, exhaust efficiency drops so that the temperature inside the furnace rises. Furthermore, as the ingot is synthesized, a deposit of SiO.sub.2 powder not captured by the synthesized surface of the ingot adheres to the exhaust piping so that the exhaust efficiency drops, also contributing to an increase in the temperature inside the furnace. As the temperature inside the furnace rises, the temperature in the vicinity of the synthesized surface of the ingot also rises, resulting in a drop in viscosity of the ingot causing the ingot to collapse. Such a temperature rise might conceivably be prevented by reducing the flow rate of the combustion gas from the burner. This arrangement, however, results in differences in the quantity of heat supplied by the burner or a temperature distribution, resulting in the generation of striae on the ingot, thus lowering the quality of the finished product.